1. Field of the Invention
The present invention relates to a semiconductor switch having a four-layer structure of p, n, p and n regions and three PN junctions, which is used as a switching element in control devices.
2. Description of the Prior Art
The examples of a semiconductor switch having a PNPN structure (hereafter referred to as a PNPN switch) are a PNPN diode having only anode and cathode electrodes, a three-terminal thyristor having a cathode gate control terminal in addition to anode and cathode terminals, and a four-terminal thyristor having both anode gate control terminal and cathode gate control terminal, besides anode and cathode. The PNPN switch is widely used in a variety of control devices, to serve as a switching element having a self-holding function.
However, the PNPN switch has a drawback that it is rendered conductive if a forward voltage is suddenly applied between the anode and cathode thereof in its cut-off state. This phenomenon is known as rate effect (also dv/dt effect) and numerous methods have been proposed to prevent the rate effect.
Most peferably practiced methods are to connect a resistor between the cathode K and the cathode gate G.sub.K of a PNPN switch, and to connect the anode gate G.sub.A of a PNPN switch with a high potential through a resistor so as to establish a reverse bias between the anode A and the anode gate G.sub.A of the PNPN switch. According to the former method, however, it is useless unless the resistor has so small a resistance that the built-in voltage between the cathode gate G.sub.K and the cathode K may not be exceeded by the voltage drop developed across the resistor when the displacement current flowing through the junction capacitance C.sub.J2 between the cathode gate and the anode gate flows through the resistor. Therefore, in order to obtain a high dv/dt withstandingness, the resistor must have a very small resistance. For example, if the junction capacitance is 2pF, the resistance must be less than 600 ohms to obtain a dv/dt withstandingness of 500v/microsec. Accordingly, those parts of the gate drive current and the holding current which flow through the resistor must be compensated through the extra supply of current. Thus, the first method has a drawback that an extra gate current of about 1mA must be supplied. On the other hand, according to the latter method, the potential must be higher than that at the anode, and moreover the second method has also a drawback that the rate effect is prevented when the anode potential shifts to a higher level while it cannot be prevented when the cathode potential is changed to a lower level.
FIGS. 1 and 2 show circuits disclosed in the U.S. Pat. No. 3,609,413 specification, which look most like the semiconductor switch according to the present invention, so far as the applicants' search is concerned. FIGS. 1 and 2 attached to the present specification corresponds to FIGS. 1 and 3 attached to the U.S. Pat. No. 3,609,413 specification, the reference numerals and characters being the same.
In order to prevent the rate effect, the junction capacitance between the emitter 19 and the base of the transistor T.sub.3 is utilized in the circuit in FIG. 1 while in the circuit in FIG. 2 the transient voltage applied to the anode is differentiated by the capacitor 34 to actuate the transistor T.sub.5. According to the circuit in FIG. 1, however, a circuit consisting of the emitter 19, the base and the emitter 18 of the transistor T.sub.3 is inserted between the anode and the cathode and an ordinary base-emitter junction breakdown voltage is at most 5- 10V. Consequently, the circuit cannot be used for voltages higher than 5V and is disadvantageous in the one of the functions of a PNPN switch, that is, the feature of high breakdown voltage in both positive and negative directions is cancelled. Moreover, if the circuits shown in FIGS. 1 and 2 are used to repeatedly fire and cut off in response to every other pulse of a pulse train with high speed repetitions, say, about 1 ms arriving at the anode, the residual charges accumulate in the capacitor 34 or the junction capacitor and the base of the transistor T.sub.3 or T.sub.5 so that the dv/dt withstandingness is considerably lowered.